The present invention relates to a field emission type cold cathode apparatus and a method of manufacturing the same.
Various possibilities are expected in a field emission type vacuum micro apparatus. For example, it may be possible to achieve a high speed response. It may also be possible to achieve improvements in resistances to radiations and to high temperatures. Further, it may be possible to achieve a self-light emitting type display device of a high precision. Under the circumstances, vigorous studies are being made in recent years on the field emission type vacuum micro apparatus. The study on the particular apparatus was motivated by a proposal on a tunnel effect vacuum triode.
However, it is a report on a cold cathode apparatus using a thin film that has caused the attentions in this technical field to be paid to the particular apparatus. A clever technique utilizing a rotary oblique vacuum vapor deposition method and an etching of a sacrificial layer is proposed in the report. To be more specific, a manufacturing method of a device, which is called a Spindt type and most widely employed nowadays, and the basic construction of the device are proposed in the report. FIGS. 1A to 1D collectively show the basic technical idea of the particular method.
As shown in the drawings, a thermal oxide film 82 is formed first on a silicon substrate 81, followed by forming a metal layer 83 made of a metal used for forming a gate electrode, e.g., molybdenum, on the thermal oxide film 82. Then, the metal layer 83 is patterned to form an opening for a gate electrode, followed by etching the oxide film 82 to form a hole 84, as shown in FIG. 1A. In the next step, a sacrificial metal, e.g., aluminum, is deposited on the metal layer 83 to form a thin sacrificial layer 85, as shown in FIG. 1B.
After formation of the sacrificial layer 85, a metal for forming an emitter, e.g., molybdenum, is formed by a rotary oblique vapor deposition method to form a metal layer 86, as shown in FIG. 1C. In this step, the metal forming the metal layer 86 is also deposited within the hole 84, with the result that the open portion of the hole 84 is gradually diminished. It follows that a molybdenum emitter 87 having a conical tip portion is formed within the hole 84. Finally, the sacrificial layer 85 is removed together with the metal layer 86 positioned on the gate electrode 83 so as to prepare a cold cathode apparatus, as shown in FIG. 1D. In this technique, however, the emitter material is limited to a material which can be used in a vapor deposition method, with the result that a nonuniformity of the element characteristics which is derived from a nonuniformity of the material tends to take place easily.
Also proposed is a technique of forming an emitter using a silicon single crystal having a high purity and good reproducibility, which is shown in FIGS. 2A to 2G.
Specifically, a silicon oxide film 92 is formed first by thermally oxidizing a surface region of a silicon substrate 91, as shown in FIG. 2A, followed by patterning the silicon oxide film 92 to form a mask 93, as shown in FIG. 2B. Then, the silicon substrate 91 is subjected to an isotropic etching using the silicon oxide mask 93, with the result that those portions of the silicon substrate 91 which are positioned below the end portions of the mask 93 are removed because of the side etching, as shown in FIG. 2C. Further, the silicon substrate 91 is subjected to an additional thermal oxidation to form a silicon oxide film 94. In this step, the narrow tip portion, which is in direct contact with the mask 93, of the silicon substrate 91 is also oxidized thermally, with the result that the silicon substrate 91 is allowed to have a sharp tip portion below the mask 93, as shown in FIG. 2D. In the next step, a gate insulating film 95 consisting of, for example, a silicon dioxide film is deposited on the entire surface with the mask 93 left unremoved, followed by forming a metal layer 96 consisting of a metal used for forming a gate electrode, e.g., molybdenum, as shown in FIG. 2E. Finally, the silicon oxide films 92 and 94 are removed by etching so as to lift off the mask 93, thereby forming a silicon emitter 97 in the center of the open portion for the gate electrode, as shown FIGS. 2F and 2G.
The conventional method shown in FIGS. 2A to 2G is excellent in that a silicon single crystal which is stable and exhibits a good reproducibility is used as a starting material, and that an emitter can be formed by self-alignment relative to a gate electrode. However, serious problems given below must be solved in putting the particular method to practical use. First of all, it is difficult to control the etching direction and rate as desired in forming the emitter by the isotropic etching of the silicon substrate. In addition, it is very difficult to form the emitter of a desired shape with a satisfactory reproducibility because the end point of the isotropic etching cannot be determined accurately. Alternatively, it is also proposed to employ an anisotropic etching of a silicon substrate for forming the emitter. However, similar problems are left unsolved in this alternative technique. Particularly, if the side etching below the oxide mask is performed to make the resultant silicon region right under the mask as thin as possible by either the isotropic etching or anisotropic etching in an attempt to form a sharply pointed emitter, the mask is likely to be collapsed. In this case, it is impossible to carry out the subsequent steps for forming the gate insulating film 95 and the gate electrode 96, quite naturally.
What should also be noted is that, in the conventional method shown in FIGS. 2A to 2G, it is necessary to form the gate insulating film 95 by a deposition method such as a sputtering or CVD method in order to ensure a sufficient height of the gate insulating film 95. Therefore, it is difficult to obtain a sufficient gate withstand voltage. Further, the open portion of the gate electrode is determined by the pattern of the mask 93, making it impossible to diminish the distance between the emitter and the gate electrode.
Incidentally, it is also known to the art that a metal layer is formed by vapor deposition in an obliquely downward direction after the step shown in FIG. 2D, followed by lifting off the mask 93. However, problems similar to those described above are also left unsolved in this method.